In the drilling for oil or gas or geothermal wells and the like, the drilling apparatus requires a fluid in the drilling region around the drill bit to remove the drill cuttings. One such drilling fluid is drilling mud which is often used when large flows of water are present in the downhole (i.e. the region where drilling takes place below ground level). It has also been found that the injection of gas into the downhole results in faster drilling rates when substantial amounts of water are not present in the downhole.
Air has been used as the principal downhole drilling fluid for low water content drilling. Straight air drilling requires only that air be compressed and circulated such that drill cuttings are lifted from the downhole free of liquids. The air can be combined with a surfactant, foaming agent, water and/or mud for different applications.
The selection of air drilling systems over mud drilling systems is based on the feasibility of drilling the hole (e.g. the presence or absence of a substantial amount of water in the downhole as well as economics). The primary advantages of straight air drilling are greatly increased penetration rates, greater bit footage and fewer downhole drilling problems.
Downhole drilling with air, however, does have a number of disadvantages, one of the most important of which is the occurrence of downhole explosions or fire due to the presence of high levels of oxygen in air. Efforts have been made to reduce the hazards of air drilling by lowering the temperature of the air or by replacing air with an inert gas.
For example, J. Q. McGuire, Jr., U.S. Pat. No. 3,612,192 discloses a process for air drilling in which the air is cooled to cryogenic temperatures. The frozen air not only reduces the threat of downhole combustion but also freezes the ground to prevent the influx of water. As is well known, cooling to cryogenic temperatures is costly and requires additional heavy equipment which may not be readily available, particularly when drilling takes place in remote locations.
Another approach to eliminating the hazards of using air as a drilling fluid is to employ an inert gas. Such systems are disclosed for example, in N. C. Wells, U.S. Pat. No. 2,786,652 and J. G. Jackson, U.S. Pat. No. 3,286,778. While the source of the inert gas (e.g. nitrogen gas) used in these systems is not set forth, it is common to use liquid nitrogen as the source of gas. Liquid nitrogen, however, is disadvantageous because it is considerably more expensive to use than air and difficult to obtain in remote locations.
An effort has been made to come up with alternative sources of nitrogen gas for use as a drilling fluid. H. E. Mallory, et al, U.S. Pat. No. 4,136,747 discloses a method of drilling in which nitrogen gas is obtained from the exhaust gas of engines. While such methods are of interest, they have not been commercialized on a large scale because of the high cost, difficulty in implementation, and technical problems such as the corrosive nature of the products of combustion.
It would therefore be desirable to devise a method by which an inert gas, typically nitrogen gas, may be conveniently and efficiently supplied to the downhole of a drilling operation in a manner which eliminates the problems associated with cryogenic nitrogen and other sources of nitrogen gas.